I want to say that you would be protected right away. I say this because when you switch pills it's usually fine, as long as you have something suppressing your hormones. I also believe there is at least a day of lag time for nexplanon to wear off. However I would recommend waiting a week just to be safe. Since you've already had sex at day five, I still think your chances of that sex resulting in pregnancy to be extremely low since you took them the same day.